The present invention relates to the structure of a semiconductor laser, and more particularly to a high-power semiconductor laser device necessary to apparatuses for consumer use.
The semiconductor laser device of this kind is used as a light source for a second harmonic generation, and is required to generate oscillation having a single transverse mode and to emit a high-power laser beam capable of forming a single lobed far field pattern. The output power of the laser device is determined by the optical power density at the facet thereof. That is, in order to increase the output power, it is required to enlarge the size of the light spot at the facet. As described in an article by D. R. Scifres et al. (Electronics Letts, Vol. 19, No. 5, 1983, pages 169 to 171), the output power can be increased by using a plurality of oscillation stripes (hereinafter simply referred to as "stripes") parallel to the direction of cavity length. In a case where the semiconductor laser is operated under pulsed current and thus it is unnecessary to take the generation of heat into consideration, all the stripes are equal in phase of oscillation to each other. FIG. 1 shows a laser beam in a case where the semiconductor laser is operated in a pulsive manner.
While, in a case where the semiconductor laser performs a CW (continuous wave) operation and thus it is necessary to take the generation of heat into consideration, phase-locked oscillation takes place only at central stripes and does not take place at outer stripes, as shown in FIG. 2. Thus, the coherency of an emitted laser beam is lowered. This phenomenon will be explained below, with reference to FIG. 3A. Now, let us consider a case where the semiconductor laser performs the CW operation. As shown in FIG. 3A, the temperature of a central portion of a stripe region is greatly increased, but the temperature of a peripheral portion of the stripe region is lower than the temperature of the central portion, because heat conducts from the peripheral portion to the outside of the stripe region. That is, non-uniform temperature distribution is generated in the stripe region. As a result, an optical property is not uniform in the stripe region. Thus, it is impossible to generate phase-locked oscillation all over the stripe region.